1. Field of the Invention
This invention generally concerns a robotic arm end effector, and, more specifically, a robotic arm end effector which provides a floating ultrasonic test probe that can be used to test baffle bolts installed on a nuclear reactor""s pressure vessel baffle. This invention further concerns a robotic arm end effector having a portion of the end effector releasably coupled to the robotic arm.
2. Background Information
Because of the radiation hazard present within the pressurized water vessel of a nuclear reactor, maintenance and testing of components within the pressurized water vessel are typically performed by remote service devices, such as robotic arms. Such a service device typically includes a robotic arm which can generally access any point within the pressure vessel. The robotic arm will be fitted with an end effector capable of performing specific maintenance or testing tasks.
One task performed remotely is the underwater testing of baffle bolts. The pressurized water vessel of a nuclear reactor houses a baffle constructed of multiple flat plates. The flat plates of the baffle are bolted together and the bolts, which are recessed in the baffle, are held in place by a lock bar welded across the bolt head and to the baffle. While the reactor is in use, the baffle bolts are exposed to stresses caused by change in temperature and vibration. Over time, the stresses may fracture the bolt and weaken the integrity of the baffle assembly. To prevent damage to the baffle assembly, baffle bolts must be regularly inspected to ensure their integrity. Baffle bolt inspection may be performed underwater using ultrasonic testing.
Presently ultrasonic testing is performed by an X-Y-Z machine having an ultrasonic probe end effector fixed to the machine. The X-Y-Z machine comprises a dolly which may be positioned at any point around the periphery of the nuclear reactor pressure vessel. The dolly supports a hydraulicly controlled shaft which descends into the water where it can be moved vertically within the pressure vessel and horizontally toward or away from the baffle. Thus, the X-Y-Z machine provides three degrees of freedom. In use, the dolly is positioned outside the pressure vessel at a point corresponding to the baffle bolts location. The shaft is then lowered to the proper depth and the probe within the end effector is moved horizontally against the baffle bolt to be tested.
To properly perform an ultrasonic test of a baffle bolt, however, an ultrasonic probe must be delivered to the baffle bolt head and oriented so the probe assembly presses firmly and evenly to the baffle bolt head. This task is made more difficult due to the presence of the lock bar. To provide a continuous mating surface with the baffle bolt head, the ultrasonic testing probe must have a recess which receives a baffle bolt head lock bar. Because a baffle bolt head lock bar may be oriented in any direction, the ultrasonic probe must be rotatable so that the recess can be aligned with the lock bar. Unless the end effector is aligned exactly with the baffle bolt, a continuous mating surface will not be formed resulting in poor testing conditions. The X-Y-Z machine described above does not provide the necessary degrees of freedom to align the ultrasonic testing probe with the baffle bolt head. However, even if the X-Y-Z machine did have two extra degrees of freedom, the alignment procedure would likely be time consuming.
Those skilled in the art will realize that the operation performed by the X-Y-Z machine could be more efficiently performed by a robotic arm. Those skilled in the art will further realize that, extreme care must be taken to avoid contacting the robot arm, or an end effector thereon, with the baffle or the pressure vessel wall.
Therefore, there is a need for an end effector that floatably supports an ultrasonic testing device so that the testing device may be pressed flush against the baffle bolt head having a lock bar even if the end effector is not exactly aligned with the baffle bolt.
Further, there is a need for a robotic arm end effector which is releasably coupled to the wrist motor of a robotic arm to be used in a water filled nuclear reactor pressure vessels so that, should the end effector catch on the edge of the baffle, the portion of the end effector that is caught on the baffle will be released so that the robotic arm, end effector and baffle will not be damaged.
These and other needs are satisfied by the invention, which is directed to an end effector for a robotic arm which is operable inside a nuclear reactor pressure vessel. The end effector incorporates a releasable coupling between the end effector wrist motor and the operable end of the end effector. This invention is further directed to an ultrasonic probe testing device mounted floatably within an end effector, so that the probe may be brought into flush contact with a baffle bolt head having a lock bar even when the end effector is not exactly aligned with the baffle bolt.
A typical robot service arm is mounted above the cylindrical pressurized water vessel. The arm may be extended to or retracted from the periphery of the pressurized water vessel. Further, the arm is rotatable 360xc2x0 so that it may be positioned above any point in the cylindrical vessel. The end of the robot arm may also travel vertically from the top of the vessel to the bottom. Thus, the arm by itself provides 3 degrees of freedom for the end effector. However, because the baffles are formed of flat plates, the baffle bolts may be as much as 90xc2x0 perpendicular to the vessel wall. Accordingly, this invention provides an extra degree of freedom through a wrist assembly. Additionally, because the lock bar on the bolt head may be oriented in any direction, the mating surface of the ultrasonic testing probe must be rotatable to match the orientation of the lock bar. However, even with these two additional degrees of freedom, it would still be time consuming to precisely align the ultrasonic testing probe with the baffle bolt head.
Therefore, this invention provides a carriage which floatably supports an ultrasonic probe with a spring biasing the probe toward the front of the car In operation, the operator is not required to precisely align the probe with the baffle bolt to be tested. Once the probe is within plus or minus 2xc2x0 of the baffle bolt to be tested, and the probe is oriented so that the groove on the probe mating surface is within plus or minus 2xc2x0 of the baffle bolt lock bar, the operator then merely advances the probe into contact with the baffle bolt where upon the probe will adjust itself within the carriage so as to provide a constant interface between the probe and the baffle bolt.
A nuclear reactor pressure vessel baffle assembly has a saw-tooth pattern along its edges. Because the saw-tooth edge of the baffle has multiple corners, care must be taken when moving the robotic arm to ensure that the end effector does not catch on the saw-tooth edge. Without a release mechanism on the end effector, the end effector, the baffle, or both could be damaged if the end effector contacted the baffle during movement of the arm.
The invention provides for a vertically oriented wrist motor assembly which may be operated to rotate an ultrasonic probe assembly, or other device, in the horizontal plane. The ultrasonic probe assembly is releasably coupled to the wrist motor drive shaft so that, in the event of contact between the probe assembly and any structure, the entire assembly member is released from the wrist drive shaft so that it may spin freely thus preventing damage to the end effector or the other structure.